This project (1) examines cancer risk in populations exposed to ionizing radiation, especially at low doses; (2) characterizes risk in terms of dose response, radiation quality, fractionation of dose, time since exposure, sex, age at exposure and at observation, and modifying influences of other environmental and host factors; and (3) examines, tests, and formulates models of radiation carcinogenesis to help define basic mechanisms. Groups studied include the Japanese A-bomb survivors, and populations with therapeutic (e.g., cervical cancer), diagnostic (e.g., tuberculosis), occupational (e.g., x-ray technologists) and environmental (e.g., to radon) exposures. Program members serve on committees advising the government and international agencies. Results of studies suggest that (1) susceptibility to radiogenic breast cancer declines with increasing age at exposure, the dose response is linear, and risk remains for at least 40 years; (2) repeated exposure to relatively low radiation doses poses some future risk of breast cancer but apparently not lung cancer; (3) high-dose radiation induces fewer leukemias than other types of exposure; cell-killing appears to play an important role in defining dose-response relationships; (4) radiotherapy for cervical cancer also increases the risk of cancers of the rectum, bladder, vagina, stomach and thyroid; (5) diagnostic x-rays may not be causally related to leukemia or lymphoma but simply related to conditions that portend their development;(6) children irradiated for benign conditions are at risk of developing thyroid and brain neoplasia; (7) radiotherapy for breast cancer did not increase the risk of leukemia; (8) radiotherapy for Hodgkin's disease increased the risk of breast cancer; (9) radiotherapy for childhood cancer increased subsequent bone cancer; (10) diagnostic xray workers in China are at high risk of leukemia; (11) approximately 9% of all lung cancer deaths may be due to indoor radon; and (12) low-dose radioactiveiodine does not appear to increase the risk of thyroid cancer.